Modern vehicles having vehicle dynamics control systems such as an ESP (Electronic Stability Program) or TCS (Traction Control System) include specially adapted brake systems. Brake systems of this type usually have multiple valves which may be used to switch between a foot brake operating mode and an automatic brake operating mode. FIG. 1 shows a hydraulic brake system 14 known from the related art which is provided to carry out a vehicle dynamics control function. Brake system 17 includes two symmetrically designed brake circuits 19a, 19b in an X or ∥ distribution pattern. Reference is thus made below only to part 19a shown on the left in FIG. 1.
The brake system includes a brake pedal 1, a brake booster 2 to which is connected a main brake cylinder 4 on which is situated a brake fluid reservoir 3. Operating brake pedal 1 produces a pressure in main brake lines 5a, 5b which acts upon brake shoes 11 of wheels 12 via a changeover valve 8a and the two intake valves 10a, 10b. The path in which pressure builds up during the operation of brake pedal 1 is identified by arrow b. A high-pressure switching valve 7a is closed in this state.
Upon the intervention of the vehicle dynamics control system, the brake pressure is automatically built up and distributed to predetermined wheels 12. For this purpose, brake system 17 includes a hydraulic pump 9a, which is activated by a control unit (not illustrated). When regulation takes place, changeover valve 8a is closed and high-pressure switching valve 7a is usually opened. Hydraulic pump 9a then delivers the hydraulic fluid along path a to brake shoes 11. The hydraulic fluid thus flows out of brake fluid reservoir 3 and passes through main brake line 5a, high-pressure switching valve 7a, an intake line 6a, hydraulic pump 9a and on through intake valves 10a, 10b to brake shoes 11. The brake pressure is modulated by intake valves 10a, 10b and discharge valves 13a, 13b, short-term pressure peaks being temporarily stored in an equalizing tank 14a. 
To prevent equalizing tank 14a from overflowing, hydraulic pump 9a regularly pumps the excess brake fluid back toward brake fluid reservoir 3. High-pressure switching valve 7a is closed for this purpose. During the return transport of the brake fluid, intake line 6a of pump 9a may be evacuated. If the main stage of high-pressure valve 7a reopens in this state, the brake fluid flows abruptly into the evacuated space of intake line 6a. This process produces a very loud noise which is irritating to the driver (known as the pressure equalization knock) and a noticeable brake pedal movement, in particular if the admission pressure is in a range from approximately 10 bar to 50 bar.
High-pressure switching valve 7a is commonly designed in two stages, a first stage and a main stage, to enable valve 7 to be opened even at high differential pressures. The differential pressure present at switching valve 7a has a closing effect on the valve. Opening the first stage slightly decreases the differential pressure so that less energy is required to open the main stage.
High-pressure switching valve 7a is customarily driven by a pulse-width-modulated voltage signal (PWM signal). To ensure that valve 7a opens safely, in particular at high differential pressures, valve 7a is activated by a 100% PWM system for a period of approximately 20 ms at the beginning of the driving action. FIG. 2a shows the variation of the PWM control signal in the case of the previous activation operation. PWM signal 20 subsequently drops, for example, to 60%, depending on the pressure, due to the thermal stressability of valve 7a (see section 23 of control signal 20). This type of activation frequently causes the main stage of switching valve 7 to open abruptly, thus resulting in the aforementioned pressure equalization knock.
FIG. 2b shows the variation of the current flowing through a coil of the valve. Current drop 24 marks the point at which the first stage of the valve opens. The main stage opens immediately thereafter, resulting in the pressure equalization knock.